1. Field of the Invention
The present invention relates to a surface acoustic wave device and, more particularly, to a surface acoustic wave device used in demodulation of data in a spread spectrum communication, and a demodulation device and a communication system using this device.
2. Description of the Related Art
A surface acoustic wave is a wave which propagates while concentrating energy near the surface of a substrate, and allows easy input/output of a signal on the surface of the substrate. Thus, a surface acoustic wave device receives a lot of attention as a signal processing device.
FIG. 1 shows a conventional surface acoustic wave device. The principle of operation of the device will be described below with reference to FIG. 1. In FIG. 1, an input electrode 2 for exciting a surface acoustic wave and an output electrode 3 for converting the surface acoustic wave into an electrical signal are formed at predetermined positions on a piezoelectric substrate 1. The output electrode 3 is constituted by a plurality of taps 4.sub.1, 4.sub.2, 4.sub.3, 4.sub.4, 4.sub.5, . . . , 4.sub.N arranged at equal intervals along a surface acoustic wave propagation path. Each of the taps 4.sub.1 to 4.sub.N comprises a so-called interdigital transducer, and consists of one or a plurality of pairs of electrode fingers.
When a signal is input to the input electrode 2, the input signal is converted into a surface acoustic wave, and the surface acoustic wave propagates toward the output electrode 3. In the output electrode 3, since the surface acoustic wave reaches the taps 4.sub.1 to 4.sub.N at predetermined time intervals, the output from the output electrode 3 becomes a synthesized output obtained by time-sampling the surface acoustic wave. The polarities of the taps 4.sub.1 to 4.sub.N can be changed according to the arrangements of their electrode fingers. As a result, a total sum of values obtained by sampling the input signal at predetermined time intervals, and adding the polarities of the corresponding taps to the sampled valued is extracted as an output. Therefore, when the polarities of the taps are properly set, the taps operate as a correlator, and when the pattern of the input signal coincides with that defined by the taps 4.sub.1 to 4.sub.N of the output electrode 3, the output becomes maximum.
Note that this surface acoustic wave device can similarly operate even when the input and output electrodes are replaced with each other. In this case, when the pattern of the input signal coincides with the tap pattern of the input electrode of the surface acoustic wave device, the output becomes maximum.
However, in the above-mentioned prior art, the taps comprise interdigital transducers having the same crossing width. Every time a surface acoustic wave passes a single tap, some components of surface acoustic wave energy are converted into an electrical signal or are reflected. For this reason, the surface acoustic wave energy propagating to the next tap is decreased. The magnitude of an electrical signal converted and extracted by each tap is decreased as the tap position is separated farther away from the input electrode. As a result, a signal obtained by synthesizing the outputs from the taps is offset from a correct correlation calculation, and signal quality deteriorates.
Since the device has only one code pattern, only a correlation signal corresponding to the code pattern on the device can be detected. For this reason, a plurality of correlation signals having different code patterns cannot be detected.